CN109768230B

CN109768230B - Lithium ion secondary battery

Info

Publication number: CN109768230B
Application number: CN201811311422.5A
Authority: CN
Inventors: 松本和明
Original assignee: Honda Motor Co Ltd
Current assignee: Honda Motor Co Ltd
Priority date: 2017-11-09
Filing date: 2018-11-06
Publication date: 2022-04-15
Anticipated expiration: 2038-11-06
Also published as: CN109768230A

Abstract

The invention provides a lithium ion secondary battery. A lithium ion secondary battery (1) is provided with a positive electrode mixture layer (3), a negative electrode mixture layer (5), and a separator (6) located between the positive electrode mixture layer (3) and the negative electrode mixture layer (5). A high-resistance layer (7) which contains a positive electrode active material and a binder or a negative electrode active material and a binder and has a volume resistivity of 10 [ omega ] cm or more is provided between the positive electrode mixture layer (3) and the separator (6) or between the negative electrode mixture layer (5) and the separator (6). The lithium ion secondary battery of the present invention can ensure electrical insulation between the positive electrode mixture layer and the negative electrode mixture layer to improve safety, while achieving a large capacity and suppressing deterioration due to charge-discharge cycles.

Description

Lithium ion secondary battery

[ technical field ]

The present invention relates to a lithium ion secondary battery.

[ background art ]

Conventionally, a lithium ion secondary battery including a positive electrode mixture layer, a negative electrode mixture layer, and a separator located between the positive electrode mixture layer and the negative electrode mixture layer has been known.

In the lithium ion secondary battery, the separator has a function of electrically insulating the positive electrode mixture layer and the negative electrode mixture layer and a function of holding an electrolytic solution, and for example, an organic polymer microporous film made of polyethylene, polypropylene, or the like is used.

Further, a lithium ion secondary battery including a porous inorganic insulating layer made of inorganic particles and containing no binder between the positive electrode mixture layer and the separator has been proposed (for example, see patent document 1).

According to the lithium ion secondary battery described in patent document 1, the safety of the battery can be improved by providing the porous inorganic insulating layer.

[ Prior art documents ]

[ patent document ]

[ patent document 1]

International publication No. 2009/081594

[ summary of the invention ]

[ problems to be solved by the invention ]

However, the lithium ion secondary battery described in patent document 1 has a disadvantage that the volumetric energy density is lowered by the presence of the porous inorganic insulating layer. In the lithium ion secondary battery described in patent document 1, since the porous inorganic insulating layer does not contain a binder, the binding force between the inorganic particles is weak, and the durability may be deteriorated when charge and discharge cycles are repeated.

The present invention has an object to eliminate the above-described problems, ensure electrical insulation between the positive electrode mixture layer and the negative electrode mixture layer, improve safety, obtain a large capacity, and suppress deterioration due to charge-discharge cycles.

[ means for solving problems ]

In order to achieve the above object, a lithium ion secondary battery according to the present invention includes a positive electrode mixture layer, a negative electrode mixture layer, and a separator interposed between the positive electrode mixture layer and the negative electrode mixture layer, and is characterized in that: a high-resistance layer containing a positive electrode active material and a binder or a negative electrode active material and a binder and having a volume resistivity of 10 Ω cm or more is provided between the positive electrode mixture layer and the separator or between the negative electrode mixture layer and the separator.

The lithium ion secondary battery of the present invention includes the high-resistance layer containing a positive electrode active material or a negative electrode active material and a binder and having a volume resistivity of 10 Ω cm or more between the positive electrode mixture layer or the negative electrode mixture layer and the separator, and therefore can ensure electrical insulation between the positive electrode mixture layer and the negative electrode mixture layer and improve safety. When the volume resistivity of the high-resistance layer is less than 10 Ω cm, electrical insulation between the positive electrode mixture layer and the negative electrode mixture layer cannot be ensured.

In the case where the high-resistance layer is provided between the positive electrode mixture layer or the negative electrode mixture layer and the separator, the high-resistance layer contains a positive electrode active material or a negative electrode active material, and electrons are supplied from the positive electrode mixture layer to the positive electrode active material on the side in contact with the positive electrode mixture layer, thereby causing a battery reaction. Similarly, on the side in contact with the negative electrode mixture layer, electrons are supplied from the negative electrode mixture layer to the negative electrode active material to generate a battery reaction, and the capacity and the volumetric energy density of the lithium ion secondary battery can be improved.

Further, since the high-resistance layer contains a binder, the binding force between the positive electrode active material and the negative electrode active material can be improved, and deterioration of the high-resistance layer due to repeated charge and discharge cycles can be suppressed, thereby improving the capacity retention rate of the lithium ion secondary battery.

In the lithium ion secondary battery of the present invention, it is preferable that the high-resistance layer contains the positive electrode active material and the binder or contains the negative electrode active material and the binder, and does not contain a conductive auxiliary agent. The high-resistance layer does not contain the conductive auxiliary agent, and therefore, conduction of electrons between the positive electrode material mixture layer and the negative electrode material mixture layer can be more reliably blocked.

The lithium ion secondary battery of the present invention may further include the high-resistance layer on the surface of the positive electrode mixture layer or the negative electrode mixture layer on the separator side.

In the case where the high-resistance layer is provided on the surface of the positive electrode mixture layer on the separator side, electrons can be reliably supplied from the positive electrode mixture layer to the positive electrode active material contained in the high-resistance layer on the side in contact with the positive electrode mixture layer, and the capacity can be reliably increased. In addition, in the case where the high-resistance layer is provided on the surface of the negative electrode mixture layer on the separator side, electrons can be reliably supplied from the negative electrode mixture layer to the negative electrode active material contained in the high-resistance layer on the side in contact with the negative electrode mixture layer, and the capacity of the entire negative electrode can be reliably increased.

The lithium ion secondary battery of the present invention may further include the high-resistance layer on the positive electrode material mixture layer side or the surface of the negative electrode material mixture layer of the separator.

When the high-resistance layer is provided on the surface of the separator on the positive electrode mixture layer side, electrons can be reliably prevented from reaching the separator from the positive electrode mixture layer, and oxidation of the separator by electrons can be suppressed. Further, by providing the high-resistance layer on the surface of the separator on the negative electrode mixture layer side, electrons can be reliably prevented from reaching the separator from the negative electrode mixture layer, and reduction of the separator by electrons can be suppressed.

In the lithium ion secondary battery of the present invention, the capacity retention rate can be improved when charge and discharge cycles are repeated by suppressing oxidation or reduction of the separator.

As described above, in the lithium ion secondary battery of the present invention, when the high-resistance layer is provided on the surface of the separator on the positive electrode mixture layer side or the negative electrode mixture layer side, the oxidation or reduction of the separator can be suppressed, but the electrolyte solution is less likely to infiltrate, and the moving distance of lithium ions in a charge-discharge cycle may become longer, as compared with a case where the separator does not include the high-resistance layer.

Therefore, in the case where the high-resistance layer is provided on the surface of the separator on the positive electrode material mixture layer side or the negative electrode material mixture layer side, the electrolyte solution impregnated into the separator preferably contains a solvent having a viscosity of 2.5mPa · s or less at 20 ℃ in an amount within a range of 60 to 95 vol%, more preferably 65 to 90 vol%, and most preferably 70 to 90 vol% of the entire electrolyte solution excluding the electrolyte salt.

In the lithium ion secondary battery of the present invention, when the separator has a high-resistance layer on the surface on the positive electrode mixture layer side or the negative electrode mixture layer side, the electrolyte solution may contain a solvent having a viscosity of 2.5mPa · s or less at 20 ℃ in an amount of 60 to 95 vol%, for example, but when the solvent is contained in an amount of 65 to 90 vol%, the electrolyte solution easily infiltrates into the separator, and the conductivity of lithium ions can be improved. As a result, in the lithium ion secondary battery of the present invention, the amount of lithium ions involved in charge and discharge increases, and the lithium ions can participate in the reaction deep inside the electrode, and the cycle capacity maintenance rate can be improved by reducing the resistance.

When the content of the solvent having a viscosity of 2.5mPa · s or less at 20 ℃ in the electrolyte solution is less than 65 vol% of the entire electrolyte solution excluding the electrolyte salt, the electrolyte solution is less likely to be impregnated into the separator, and the moving distance of lithium ions in a charge-discharge cycle becomes long, and therefore the cycle capacity retention rate may not be sufficiently increased. In addition, when the content of the solvent having a viscosity of 2.5mPa · s or less at 20 ℃ in the electrolyte solution exceeds a range of more than 90 vol% of the entire electrolyte solution excluding the electrolyte salt, dissociation of the lithium salt in the electrolyte solution is difficult to proceed, and the cycle capacity retention rate may not be sufficiently improved.

In the lithium ion secondary battery of the present invention, a combination of the binder and the electrolyte used for the high-resistance layer is selected, and the binder absorbs the electrolyte and swells, so that voids in the high-resistance layer are reduced and clogging occurs, and therefore, conduction of lithium ions may be inhibited.

Therefore, in the lithium ion secondary battery of the present invention, when the high-resistance layer is provided on the surface of the separator on the positive electrode mixture layer side or the negative electrode mixture layer side, the electrolyte contains a solvent having a viscosity of 2.5mPa · s or less at 20 ℃ in an amount in the range of 70 to 90 vol%, and thus occurrence of clogging can be suppressed, conductivity of lithium ions can be improved, and discharge rate characteristics can be improved.

In the lithium ion secondary battery of the present invention, it is preferable that the electrolytic solution is in a gel state. By forming the electrolytic solution into a gel state, swelling of the binder can be suppressed.

In the lithium ion secondary battery of the present invention, the electrolyte solution preferably contains an electrolyte salt at a concentration of 0.1 to 3.5 mol/L, and more preferably contains an electrolyte salt at a concentration of 1.0 to 2.5 mol/L. The electrolyte contains electrolyte salt with the concentration of 1.0-2.5 mol/L, so that the amount of the solvent is relatively reduced, and the swelling of the adhesive can be inhibited.

[ description of the drawings ]

FIG. 1 is an explanatory cross-sectional view showing the structure of a lithium-ion secondary battery of the present invention.

Fig. 2 is a graph showing an example of cycle characteristics (a relationship between capacity and the number of charge and discharge cycles) of the lithium ion secondary battery of the present invention.

Fig. 3 is a graph showing an example of the relationship between the capacity retention rate and the number of charge/discharge cycles in the lithium-ion secondary battery of the present invention.

Fig. 4 is a graph showing an example of the relationship between the capacity retention rate and the discharge rate in the lithium-ion secondary battery of the present invention.

Fig. 5 is a graph showing another example of the cycle characteristics (the relationship between the capacity and the number of charge and discharge cycles) of the lithium ion secondary battery of the present invention.

Fig. 6 is a graph showing another example of the relationship between the capacity retention rate and the number of charge and discharge cycles in the lithium-ion secondary battery of the present invention.

Fig. 7 is a graph showing another example of the relationship between the capacity retention rate and the number of charge and discharge cycles in the lithium-ion secondary battery according to the present invention.

[ detailed description of the invention ]

Next, embodiments of the present invention will be described in further detail with reference to the drawings.

As shown in fig. 1, a lithium ion secondary battery 1 of the present embodiment includes a separator 6 impregnated with an electrolyte between a positive electrode mixture layer 3 formed on a positive electrode current collector 2 and a negative electrode mixture layer 5 formed on a negative electrode current collector 4, and a high-resistance layer 7 containing a positive electrode active material and a binder and not containing a conductive additive between the positive electrode mixture layer 3 and the separator 6.

In addition, the lithium-ion secondary battery 1 of the present embodiment may include the high-resistance layer 7 between the negative electrode mixture layer 5 and the separator 6 instead of the configuration in which the high-resistance layer 7 is provided between the positive electrode mixture layer 3 and the separator 6 shown in fig. 1. In this case, the high-resistance layer 7 contains the negative electrode active material and the binder without containing the conductive aid.

As the positive electrode current collector 2, for example, a metal foil of aluminum, stainless steel, nickel, titanium, or the like can be used. The metal foil may have a thickness in the range of 3 to 500 μm, for example.

The positive electrode mixture layer 3 contains a positive electrode active material, a binder, and a conductive auxiliary agent. Examples of the positive electrode active material include: LiMnO₂、LixMn₂O₄(0＜x＜2)、Li₂MnO₃、LixMn_1.5Ni_0.5O₄(0 < x < 2), and the like, and has a layered structure of lithium manganate or spinel; LiCo₂O₂、LiNiO₂Or a compound in which a part of the transition metal is replaced with another metal; LiNi_1/3Co_1/3O₂Lithium transition metal oxides having no more than half of the total of the specific transition metals; compounds having more excess Li than the stoichiometric composition in these lithium transition metal oxides; LiFePO₄And compounds having an olivine structure. In addition, as the positive electrode active material, a material In which a part of the metal of these metal oxides is replaced with Al, Fe, P, Ti, Si, Pb, Sn, In, Bi, Ag, Ba, Ca, Hg, Pd, Pt, Te, Zn, La, or the like is used. Further, an olivine compound may be used.

The positive electrode active material is particularly preferably Li_αNi_βCo_γAl_δO₂(1. ltoreq. alpha. ltoreq.2, beta + gamma + delta. ltoreq.1, beta. ltoreq.0.7, gamma. ltoreq.0.01) or Li_αNi_βCo_γMn_δO₂(alpha is more than or equal to 1 and less than or equal to 1.2, beta is more than or equal to 0.01, and gamma is less than or equal to 0.01). As the positive electrode active material, iron sulfide, iron disulfide, sulfur, polysulfide, Li3VO4, or the like may be used.

The positive electrode active material may be used alone or in combination of two or more compounds.

Examples of the binder include polyvinylidene fluoride (PVDF), Polyimide (PI), polyamide imide (PAI), Polytetrafluoroethylene (PTFE), polyacrylic acid, carboxymethylcellulose (CMC), Styrene Butadiene Rubber (SBR), and the like. Examples of the conductive assistant include carbon black, acetylene black, fullerene, carbon nanotube, and carbon nanofiber.

The binder and the conductive assistant may be used alone or in combination of two or more of the above materials.

As the negative electrode current collector 4, for example, a metal foil of aluminum, copper, stainless steel, titanium, nickel, or the like can be used. The metal foil may have a thickness in the range of 3 to 500 μm, for example.

The negative electrode mixture layer 5 contains a negative electrode active material, a binder, and a conductive auxiliary agent. Examples of the negative electrode active material include artificial graphite, natural graphite, hard carbon, soft carbon, silicon oxide, tin oxide, silver, aluminum, zinc, lead, germanium, lithium, and alloys thereof.

Examples of the binder include polyvinylidene fluoride (PVDF), Polyimide (PI), polyamide imide (PAI), Polytetrafluoroethylene (PTFE), polyacrylic acid, carboxymethyl cellulose (CMC), Styrene Butadiene Rubber (SBR), and polymethyl methacrylate.

In addition, as the adhesive, acrylonitrile, acrylic acid can also be used, in this case can be added with a polymerization initiator or heating and polymerization. Examples of the polymerization initiator include azobis (2, 4-dimethylvaleronitrile) and the like. When heating is performed, the temperature is preferably set to a range of 45 to 75 ℃.

Examples of the conductive aid include carbon black, acetylene black, fullerene, carbon nanotube, and carbon nanofiber.

The negative electrode active material, the binder, and the conductive assistant may be used singly or in combination of two or more of the above materials.

When a material such as artificial graphite, natural graphite, hard carbon, soft carbon, tin oxide, silver, aluminum, zinc, lead, germanium, or lithium is used as the negative electrode active material, the negative electrode mixture layer 5 may not contain the conductive auxiliary agent because the material also functions as the conductive auxiliary agent. The negative electrode mixture layer 5 may contain an additive such as a thickener.

Examples of the separator 6 include an organic polymer film made of polyethylene, polypropylene, polyvinylidene fluoride, cellulose, glass fiber, polyimide, alumina, silica, or the like, or an inorganic material. The organic polymer film is preferably a microporous film, and for example, a film having a thickness in the range of 3 to 100 μm can be used.

The electrolyte solution to be impregnated in the separator 6 may be an electrolyte solution in which an electrolyte salt is dissolved in a non-aqueous solvent at a concentration in the range of 0.1 to 3.5 mol/L, and preferably contains the electrolyte salt at a concentration in the range of 1.0 to 2.5 mol/L.

Examples of the non-aqueous solvent include Vinylene Carbonate (VC), Vinyl Ethylene Carbonate (VEC), Ethylene Carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), methylethyl carbonate (EMC), Propylene Carbonate (PC), dimethoxyethane having an ether group, diglyme, tetraglyme, triglyme, (anhydrous) succinic acid, (anhydrous) maleic acid, γ -butyrolactone, γ -valerolactone, ethylene sulfite, sulfolane, ionic liquids, phosphate esters, borate esters, acetonitrile, phosphazene, and the like, and substances obtained by fluorinating hydrogen groups in a part of these substances.

Alternatively, a gelling agent such as a polymerization initiator or a polymer may be added to the electrolyte solution to gel the electrolyte solution. Examples of the polymerization initiator or polymer as the gelling agent include polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), (poly) acrylonitrile, (poly) acrylic acid, and polymethyl methacrylate, but are not limited thereto. In addition, a crosslinking agent (crosslinking agent) may be added to these gelling agents. In order to gel the electrolyte, the entire battery may be heated after the battery is manufactured, and the electrolyte may be thermally polymerized for use.

The electrolyte salt includes, for example, LiPF₆、LiAsF₆、LiAlCl₄、LiClO₄，LiBF₄、LiSbF₆、Li₂SO₄、Li₃PO₄、Li₂HPO₄、LiH₂PO₄，LiCF₃SO₃、Li₄F₉SO₃LiN (FSO) comprising imide anion₂)₂、LiN(CF₃SO₂)₂、LiN(C₂F₅SO₂)₂、LiN(CF₃SO₂)(C₂F₅SO₂)、LiN(CF₃SO₂)(C₄F₉SO₂) And LiN (CF) having a 5-membered ring structure₂SO₂)₂(CF₂) LiN (CF) having a 6-membered ring structure₂SO₂)₂(CF₂)₂And the like lithium salts.

Further, the lithium salt may be LiPF₆LiPF in which at least 1 fluorine atom is substituted by fluoroalkyl group₅(CF₃)、LiPF₅(C₂F₅)、LiPF₅(C₂F₅)、LiPF₅(C₃F₇)、LiPF₄(CF₃)₂、LiPF₄(CF₃)(C₂F₅)、LiPF₃(CF₃)₃And the like. The nonaqueous solvent and the electrolyte salt may be used singly or in combination of two or more of the above materials.

The high-resistance layer 7 contains a positive electrode active material or a negative electrode active material and a binder, and does not contain a conductive auxiliary agent. As the positive electrode active material or the negative electrode active material, for example, the same material as the positive electrode active material or the negative electrode active material, and Li may be used₄Ti₅O₁₂(LTO). In addition, a solid electrolyte containing lithium or the like may be usedMaterial of (2), CoO, NiO, MnO₂、FePO₄、MnO₃、TiO₂、Al₂O₃、SiO₂Etc. do not contain lithium oxide.

As the binder, for example, the same binder as used for the positive electrode mixture layer 3 or the negative electrode mixture layer 5 can be used. In this case, when a binder dissolved or dispersed in a solvent system such as N-methyl-2-pyrrolidone (NMP) is used for the positive electrode mixture layer 3, it is preferable to use a binder dissolved or dispersed in an aqueous solvent for the high-resistance layer 7.

The positive electrode active material, the negative electrode active material, and the binder may be each one of the above materials, or two or more of the above materials may be used in combination.

Since the lithium-ion secondary battery 1 of the present embodiment includes the high-resistance layer 7 between the positive electrode mixture layer 3 and the separator 6 or between the negative electrode mixture layer 5 and the separator 6, electron conduction between the positive electrode mixture layer 3 and the negative electrode mixture layer 5 can be reliably blocked. In addition, when the high-resistance layer 7 is provided between the positive electrode mixture layer 3 and the separator 6, since the high-resistance layer 7 contains the positive electrode active material, electrons are supplied from the positive electrode mixture layer 3 to the positive electrode active material on the side in contact with the positive electrode mixture layer 3 to cause a battery reaction, and the capacity and the volumetric energy density of the lithium ion secondary battery 1 can be improved. In addition, when the high-resistance layer 7 is provided between the negative electrode mixture layer 5 and the separator 6, since the high-resistance layer 7 contains the negative electrode active material, electrons are supplied to the high-resistance layer to cause a battery reaction, and the capacity and the volumetric energy density of the lithium ion secondary battery 1 can be improved.

Further, since the high-resistance layer 7 contains a binder, the binding force between the positive electrode active materials and between the negative electrode active materials can be improved, and deterioration of the high-resistance layer 7 due to repeated charge and discharge cycles can be suppressed, so that the capacity retention rate of the lithium ion secondary battery 1 can be improved.

The lithium-ion secondary battery 1 of the present embodiment can be manufactured, for example, in the following manner.

First, the positive electrode active material, the binder, and the conductive additive are mixed at a predetermined ratio to prepare a slurry for a positive electrode mixture layer. Next, the slurry for a positive electrode mixture layer is applied to the positive electrode current collector 2 to form the positive electrode mixture layer 3.

Then, the positive electrode active material and the binder are mixed at a predetermined ratio to prepare a slurry for a high resistance layer. Next, the slurry for a high resistance layer is applied to the positive electrode mixture layer 3 to form a high resistance layer 7. The slurry for a high resistance layer may contain an additive such as a thickener, but does not contain a conductive assistant.

Next, the positive electrode current collector 2 on which the positive electrode mixture layer 3 and the high-resistance layer 7 are formed is cut into a predetermined shape to form a positive electrode.

Then, the negative electrode active material, the binder, and the conductive assistant are mixed at a predetermined ratio to prepare a slurry for a negative electrode mixture layer. When a carbonaceous material is used as the negative electrode active material, the slurry for the negative electrode mixture layer may not contain the conductive auxiliary agent. Next, the slurry for a negative electrode mixture layer is applied to the negative electrode current collector 4 to form the negative electrode mixture layer 5. Next, the negative electrode current collector 4 on which the negative electrode mixture layer 5 is formed is cut into a predetermined shape to form a negative electrode.

Next, the organic polymer film is cut into a predetermined shape, and the separator 6 is formed by impregnating the electrolyte solution.

The lithium ion secondary battery 1 of the present embodiment, for example, in the shape of a coin battery, can be obtained by disposing and overlapping the separator 6 between the positive electrode and the negative electrode.

In the above manufacturing method, the high-resistance layer 7 is formed by applying the slurry for a high-resistance layer on the positive electrode mixture layer 3, but the high-resistance layer 7 may be formed by applying a slurry for a high-resistance layer in which the negative electrode active material and the binder are mixed at a predetermined ratio on the negative electrode mixture layer 5.

In the lithium-ion secondary battery 1 of the present embodiment, when the slurry for a high-resistance layer is applied to the positive electrode mixture layer 3 and the high-resistance layer 7 is provided on the surface of the positive electrode mixture layer 3 on the separator 6 side, electrons can be reliably supplied from the positive electrode mixture layer 3 to the positive electrode active material of the high-resistance layer 7, and the capacity can be reliably increased. In the lithium-ion secondary battery 1 of the present embodiment, when the slurry for a high-resistance layer is applied to the negative electrode mixture layer 5 and the high-resistance layer 7 is provided on the surface of the negative electrode mixture layer 5 on the separator 6 side, electrons can be reliably supplied from the negative electrode mixture layer 3 to the negative electrode active material of the high-resistance layer 7, and the capacity can be reliably increased.

In the lithium-ion secondary battery 1 of the present embodiment, the high-resistance layer 7 may be formed by applying a slurry for a high-resistance layer in which the positive electrode active material and the binder are mixed at a predetermined ratio to the surface of the separator 6 on the positive electrode mixture layer 3 side, or the high-resistance layer 7 may be formed by applying a slurry for a high-resistance layer in which the negative electrode active material and the binder are mixed at a predetermined ratio to the surface of the separator 6 on the negative electrode mixture layer 5 side.

In the lithium-ion secondary battery 1 of the present embodiment, when the high-resistance layer 7 is provided on the surface of the separator 6 on the positive electrode mixture layer 3 side, electrons can be reliably prevented from reaching the separator 6 from the positive electrode mixture layer 3, oxidation of the separator 6 by electrons can be suppressed, and the capacity retention rate can be improved when charge and discharge cycles are repeated. In the lithium-ion secondary battery 1 of the present embodiment, when the high-resistance layer 7 is provided on the surface of the separator 6 on the negative electrode mixture layer 5 side, electrons can be reliably prevented from reaching the separator 6 from the negative electrode mixture layer 5, reduction of the separator 6 by electrons can be suppressed, and the capacity retention rate can be improved when charge and discharge cycles are repeated.

On the other hand, as described above, in the lithium ion secondary battery 1 of the present embodiment, when the high-resistance layer 7 is provided on the surface of the separator 6 on the positive electrode mixture layer 3 side or the negative electrode mixture layer 5 side, the electrolyte solution is less likely to infiltrate, and the moving distance of lithium ions in the charge-discharge cycle may become longer, as compared with the case where the separator 6 does not include the high-resistance layer 7.

Therefore, in the lithium ion secondary battery 1 of the present embodiment, when the high-resistance layer 7 is provided on the surface of the separator 6 on the positive electrode mixture layer 3 side or the negative electrode mixture layer 5 side, the solvent having a viscosity of 2.5mPa · s or less at 20 ℃ is preferably contained in the electrolyte solution impregnated into the separator 6 in an amount within a range of 60 to 95 vol% of the entire electrolyte solution excluding the electrolyte salt, more preferably 65 to 90 vol%, and most preferably 70 to 90 vol%.

In the lithium ion secondary battery 1 of the present embodiment, when the high-resistance layer 7 is provided on the surface of the separator 6 on the positive electrode mixture layer 3 side or the negative electrode mixture layer 5 side, the solvent having a viscosity of 2.5mPa · s or less at 20 ℃ is contained in an amount within a range of 65 to 90 vol% of the entire electrolyte solution excluding the electrolyte salt, so that the separator 6 can be easily impregnated with the electrolyte solution, and the lithium ion conductivity can be improved. As a result, in the lithium ion secondary battery 1 of the present embodiment, the amount of lithium ions involved in charge and discharge increases, and the lithium ions can participate in the reaction deep inside the electrode, and the cycle capacity retention rate can be improved by reducing the resistance.

Further, by containing a solvent having a viscosity of 2.5mPa · s or less at 20 ℃ in an amount of 70 to 90 vol% of the entire electrolyte excluding the electrolyte salt, swelling of the binder can be suppressed, clogging can be prevented, and lithium ion conductivity can be improved, so that discharge rate characteristics (rate characteristics) can be improved.

Examples of the solvent having a viscosity of 2.5 mPas or less at 20 ℃ include dimethyl carbonate (0.59 mPas), methyl ethyl carbonate (0.65 mPas), diethyl carbonate (0.75 mPas), dimethyl sulfoxide (1.99 mPas), acetonitrile (0.35 mPas), dimethoxyethane (0.45 mPas), dioxolane (0.78 mPas), ethylene glycol dimethyl ether (0.57 mPas), propylene glycol dimethyl ether (0.57 mPas), triethylene glycol dimethyl ether (2.3 mPas), diethylene glycol dibutyl ether (2.47 mPas), gamma butyrolactone (1.75 mPas), propionic anhydride (1.14 mPas), diethyl ether (0.25 mPas), ether (0.24 mPas), acetone (0.3 mPas), ethyl acetate (0.38 mPas), ethyl acetate (0.43 mPas), hexane (0.3 mPas), methylcyclohexane (0.65 mPas), cyclopentane (0.43 mPas), Cyclohexane (0.98 mPas), cycloheptane (1.02 mPas), isopropyl ether (0.38 mPas), methyl ethyl ketone (0.38 mPas), tetrahydrofuran (0.46 mPas), toluene (0.55 mPas), octane (0.56 mPas), chlorobenzene (1.01 mPas), 1, 2, 4-trichlorobenzene (1.89 mPas), o-dichlorobenzene (1.26 mPas), benzene (0.63 mPas), acetic acid (1.1 mPas), trifluoroethylcarbonate (1.2 mPas), trimethyl phosphate (2.25 mPas), triethyl phosphate (2.1 mPas), 1, 2, 2-tetrafluoroethyl-2, 2, 2-trifluoroethyl ether (0.65 mPas), dimethyl succinate (2.3 mPas), diethyl succinate (2.45 mPas), methyl nonafluorobutyl ether (0.38 mPas), tridecyl ether (0.7 mPas), methyl hexyl ether (0.7 mPas), methyl fluorobutyl ether (0.7 mPas), Hydrofluoroether (0.38 mPas), bis (2, 2, 2-trifluoroethyl) ether (0.6 mPas), 2, 2, 3, 3, 3-pentafluoropropyldifluoromethyl ether (1.49 mPas), 2, 2, 3, 3, 3-pentafluoropropyl-1, 1, 2, 2-tetrafluoroethyl ether (1.57 mPas), 1, 2, 2-tetrafluoroethyl methyl ether (1.29 mPas), 1, 2, 2-tetrafluoroethyl ether (1.2 mPas), 1, 2, 2-tetrafluoroethyl-2, 2, 2-trifluoroethyl ether (1.5 mPas), 1, 2, 2-tetrafluoroethyl-2, 2, 3, 3-tetrafluoropropyl ether (1.53 mPas), hexafluoroisopropylmethyl ether (1.5 mPas), 1, 3, 3, 3-pentafluoro-2-trifluoromethylpropyl ether (1.5 mPas), 1, 1, 2, 3, 3, 3-hexafluoropropyl methyl ether (1.5 mPas), 1, 2, 3, 3, 3-hexafluoropropyl ethyl ether (1.3 mPas), 2, 3, 4, 4, 4-hexafluorobutyl difluoromethyl ether (1.5 mPas), methyl trifluoroacetate (1.3 mPas), ethyltrifluoroacetate (1.3 mPas), methylperfluoropropyl ester (1.4 mPas), ethylperfluoropropyl ester (1.2 mPas), methylperfluorobutyric acid ester (1.5 mPas), ethylperfluorobutyric acid ester (1.4 mPas), methyl acetate (1.3 mPas), difluoroacetic acid ester (1.1 mPas), ethyl 5H-octafluoropentaphenol (1.5 mPas), ethyl 7H-dodecafluoropentaphenol (1.6 mPas), methyl 2-trifluoromethyl-3, 3, 3-trifluoro-methyl ester (1.6 mPas), Methyl 3, 3, 3-trifluoropropionate (0.83 mPas), acetic acid 222-trifluoroethyl ester (0.83 mPas), 2, 2, 2-trifluoromethylmethylethyl ester (0.99 mPas), difluoroethylcarbonate (2.49 mPas), and the like.

Examples of the solvent that is not contained in the solvent having a viscosity of 2.5mPa · s or less at 20 ℃ include ethylene carbonate (solid), propylene carbonate (2.53mPa · s), sulfolane (solid), fluoroethylene carbonate (4.1mPa · s), tris (2, 2, 2-trifluoroethyl) phosphate (4.0mPa · s), and the like.

Next, examples of the present invention and comparative examples are shown.

[ examples ]

[ example 1]

In this example, first, LiNi as a positive electrode active material was mixed at a mass ratio of 92:4:4_1/3Co_1/ ₃Mn_1/3O₂Polyvinylidene fluoride (PVDF) as a binder and acetylene black as a conductive aid were used to prepare a slurry for a positive electrode mixture layer. Next, the slurry for a positive electrode mixture layer was applied by a doctor blade method (doctor blade method) to an aluminum foil having a thickness of 20 μm serving as the positive electrode current collector 2, thereby forming a positive electrode mixture layer 3 having a thickness of 30 μm. The volume resistivity of the positive electrode mixture layer 3 was measured, and found to be 3.1. omega. cm.

Subsequently, LiMn as a positive electrode active material was mixed at a mass ratio of 98:1:1₂O₄Styrene Butadiene Rubber (SBR) as a binder and carboxymethyl cellulose (CMC) as a thickener, a slurry for a high resistance layer was prepared. Next, the slurry for a high resistance layer was applied to the positive electrode mixture layer 3 by a doctor blade method to form a high resistance layer 7 having a thickness of 25 μm. The volume resistivity of the high-resistance layer 7 was measured to be 1.5 × 10⁴Ωcm。

Next, the positive electrode current collector 2 on which the positive electrode mixture layer 3 and the high-resistance layer 7 were formed was punched out into a disk shape having a diameter of 14mm to form a positive electrode.

Next, hard carbon as the negative electrode active material and PVDF as the binder were mixed at a mass ratio of 95:5 to prepare a slurry for a negative electrode mixture layer. Then, the slurry for a negative electrode mixture layer was applied by a doctor blade method to a copper foil having a thickness of 15 μm as a negative electrode current collector 4, thereby forming a negative electrode mixture layer 5 having a thickness of 32 μm. Next, the negative electrode current collector 4 on which the negative electrode mixture layer 5 was formed was punched out into a disk shape having a diameter of 15mm to form a negative electrode. The volume resistivity of the negative electrode mixture layer 5 was 0.1 Ω cm.

Then, the polyethylene film was punched out into a disc shape having a diameter of 19.5mm, and immersed in the electrolyte solution to form the separator 6. The electrolyte is used in a volume ratio of 30:30:40LiPF as an electrolyte salt was dissolved in a mixed solvent of ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate at a concentration of 1.0mol/L₆And (3) obtaining a solution.

Then, a separator 6 was disposed between the positive electrode and the negative electrode, and the two were stacked to obtain a button cell type lithium ion secondary battery 1.

Next, a charge/discharge test was performed using the lithium ion secondary battery 1 obtained in this example. In the above charge and discharge test, the operation of charging to 4.15V at 1C and discharging to 2.5V at 1C was set as 1 cycle, and cycle performance (relationship of capacity with respect to the number of charge and discharge cycles) and the capacity retention rate with respect to the number of charge and discharge cycles were measured by repeating the operation for 200 cycles. The cycle performance is shown in fig. 2, and the capacity retention rate is shown in fig. 3.

[ example 2]

In this example, a button cell type lithium ion secondary battery 1 was produced in exactly the same manner as in example 1, except that the slurry for the high resistance layer was not applied to the positive electrode mixture layer 3 and the high resistance layer 7 was formed by applying the slurry to the surface of the separator 6 on the positive electrode mixture layer 3 side.

Next, a charge and discharge test was performed in exactly the same manner as in example 1, except that the lithium-ion secondary battery 1 obtained in this example was used. The cycle performance is shown in FIG. 2, and the capacity retention rate is shown in FIG. 3.

[ comparative example 1]

In this comparative example, a button cell type lithium ion secondary battery 1 was obtained in the same manner as in example 1, except that the high-resistance layer 7 was not formed at all.

Next, a charge and discharge test was performed in exactly the same manner as in example 1, except that the lithium-ion secondary battery 1 obtained in this comparative example was used. The cycle performance is shown in FIG. 2, and the capacity retention rate is shown in FIG. 3.

As is apparent from fig. 2, the lithium ion secondary batteries 1 of examples 1 and 2, which have the high-resistance layer 7 between the positive electrode mixture layer 3 and the separator 6, have improved capacity as compared with the lithium ion secondary battery 1 of comparative example 1, which does not have the high-resistance layer 7 at all.

As is apparent from fig. 3, the lithium ion secondary batteries 1 of examples 1 and 2, which have the high-resistance layer 7 between the positive electrode mixture layer 3 and the separator 6, have a higher capacity retention rate than the lithium ion secondary battery 1 of comparative example 1, which does not have the high-resistance layer 7 at all. In addition, according to the lithium ion secondary battery 1 of example 2 in which the high-resistance layer 7 is provided on the surface of the separator 6 on the positive electrode mixture layer 3 side, the capacity retention rate is higher than that of the lithium ion secondary battery 1 of example 1 in which the high-resistance layer 7 is provided on the surface of the positive electrode mixture layer 3 on the separator side. The reason why the capacity retention rate of the lithium ion secondary battery 1 of example 2 is higher than that of the lithium ion secondary battery 1 of example 1 is considered to be that oxidation of the separator 6 by electrons can be suppressed as a result of reliably preventing electrons from reaching the separator 6 from the positive electrode mixture layer 3 by the high-resistance layer 7.

[ example 3]

In the present embodiment, first, LiNi is used_0.5Co_0.2Mn_0.3O₂Except for the above, in the same manner as in example 1, the positive electrode mixture layer 3 was formed on the positive electrode current collector 2, and the positive electrode current collector 2 on which the positive electrode mixture layer 3 was formed was punched out to a size of 3cm × 4cm to form a positive electrode. Next, in exactly the same manner as in example 1, the negative electrode mixture layer 5 was formed on the negative electrode current collector 4, and the negative electrode current collector 4 on which the negative electrode mixture layer 5 was formed was punched out to a size of 3.4cm × 4.4cm, to form a negative electrode.

Subsequently, LiMn as a positive electrode active material was mixed in a mass ratio of 99: 1₂O₄And carboxymethyl cellulose (CMC) as a binder, and a slurry for a high resistance layer is prepared. Then, the slurry for a high resistance layer was applied to the surface of the polyethylene film by a doctor blade method to form a high resistance layer 7 having a thickness of 8 μm. The volume resistivity of the high-resistance layer 7 was measured to be 2.5 × 10³Omega cm. Next, the polyethylene film on which the high-resistance layer 7 was formed was punched out to a size of 4cm × 5cm, and was immersed in the same electrolyte as in example 1, thereby forming the separator 6.

Next, a laminate battery (laminate cell) was formed by disposing and laminating a separator 6 between the positive electrode and the negative electrode so that the high-resistance layer 7 was on the positive electrode mixture layer 3 side, thereby producing a laminate lithium ion secondary battery 1.

Then, a discharge rate characteristic test was performed using the lithium ion secondary battery 1 obtained in this example. In the discharge rate characteristic test, when the battery is charged to 4.15V at 0.2C and discharged to 2.5V at a predetermined discharge rate, the discharge rate is discharged so as to increase the discharge rate in the order of 0.25C, 0.5C, 0.75C, 1.0C, 1.5C, 2.0C, 2.5C, 3.0C, 3.5C, and 4.0C. The capacity at the time of discharge at a discharge rate of 0.25C was set to 100, and the capacity retention rate at each discharge rate was measured with respect to this value. The results are shown in FIG. 4.

[ example 4]

In this example, a stacked lithium ion secondary battery 1 was produced in exactly the same manner as in example 3, except that the solvent of the electrolyte was prepared as a mixed solvent in which ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate were mixed in a volume ratio of 25:35: 40.

Next, a discharge rate characteristic test was performed in the same manner as in example 3, except that the lithium-ion secondary battery 1 obtained in this example was used. The results are shown in FIG. 4.

[ example 5]

In this example, a solvent of an electrolyte was prepared as a mixed solvent in which ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate were mixed at a volume ratio of 25:35:40, 4 mass% of polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) was added to the mixed solvent, and after injection, the battery was sealed and left to stand at 60 ℃ for 1 hour to gel and form an electrolyte in a gel state. A stacked lithium ion secondary battery 1 was produced in the same manner as in example 3, except that the gel-state electrolyte solution prepared as described above was used.

[ example 6]

In this example, a stacked lithium ion secondary battery 1 was produced in the same manner as in example 3, except that the solvent of the electrolyte solution was prepared as a mixed solvent in which ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate were mixed at a volume ratio of 25:35:40, and the concentration of the electrolyte salt was adjusted to 2.0 mol/L.

[ example 7]

In this example, a stacked lithium ion secondary battery 1 was obtained in exactly the same manner as in example 3, except that the solvent of the electrolyte was prepared as a mixed solvent in which ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate were mixed at a volume ratio of 40:30: 30.

Next, a discharge rate characteristic test was performed in the same manner as in example 3, except that the lithium-ion secondary battery 1 obtained in this comparative example was used. The results are shown in FIG. 4.

[ comparative example 2]

In this comparative example, a stacked lithium ion secondary battery 1 was produced in exactly the same manner as in example 3, except that the high-resistance layer 7 was not formed at all.

As is clear from fig. 4, according to the lithium ion secondary batteries 1 of examples 3 to 7 having the high-resistance layer 7 on the surface of the separator 6 on the positive electrode mixture layer 3 side, the capacity retention rate at each discharge rate of 0.5 to 4.0C is higher than that of the lithium ion secondary battery 1 of comparative example 2 having no high-resistance layer 7 at all. This is considered to be because the viscosity of the electrolytic solution is decreased, and the lithium ion conductivity in the high-resistance layer is improved.

It is also understood that the lithium ion secondary battery 1 of example 5 in which the electrolyte solution was gelled had a higher capacity retention rate at each discharge rate of 0.5 to 4.0C than the lithium ion secondary batteries 1 of examples 3, 4, and 7. This is presumably because gelation of the electrolyte solution suppresses swelling of the binder, prevents clogging, and improves lithium ion conductivity.

In addition, according to the lithium ion secondary battery 1 of example 6 in which the concentration of the electrolyte salt was set to 2.0mol/L, the capacity retention rate at each discharge rate of 0.5 to 4.0C was higher than that of the lithium ion secondary batteries 1 of examples 3 to 5 and 7. This is presumably because the amount of solvent in the electrolytic solution is reduced by increasing the electrolyte salt concentration, and the binder is less likely to swell, so that clogging can be prevented from occurring, and the lithium ion conductivity can be improved.

[ reference example 1]

In this reference example, first, an aqueous solution in which carboxymethyl cellulose (CMC) as a binder was dissolved in an amount of 1.2 mass% was applied to a glass plate, and then water was evaporated to prepare a film composed only of carboxymethyl cellulose (CMC) as a binder. Next, the film was punched out into a disk shape having a diameter of 20mm, and the mass A was measured.

Then, the disk-shaped membrane punched out was immersed in a mixed solvent in which ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate were mixed at a volume ratio of 30:30:40, and immersed in LiPF dissolved as an electrolyte salt at a concentration of 1.0mol/L₆The electrolyte solution of (4) was left at 45 ℃ for 10 hours. Then, the film was taken out from the electrolyte, and after excess electrolyte was wiped off, the mass B of the film was measured. Then, the swelling degree of the film was calculated from the following formula (1) by comparing the mass a before immersion in the electrolyte solution and the mass B after immersion. The results are shown in Table 1.

Degree of swelling (%) { (mass B-mass a)/mass a } × 100 … (1)

[ reference example 2]

In this reference example, ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate were mixed at a volume ratio of 25:35:40, and LiPF as an electrolyte salt was dissolved at a concentration of 1.0mol/L₆The swelling degree of the film was calculated in exactly the same manner as in reference example 1 except for the electrolyte solution (b). The results are shown in Table 1.

[ reference example 3]

In this reference example, mixing at a volume ratio of 20:30:20:30 was usedEthylene carbonate, ethylmethyl carbonate, dimethyl carbonate and bis (2, 2, 2-trifluoroethyl) ether, in which LiPF as an electrolyte salt was dissolved at a concentration of 1.0mol/L₆The swelling degree of the film was calculated in exactly the same manner as in reference example 1 except for the electrolyte solution (b). The results are shown in Table 1.

[ reference example 4]

In this reference example, the swelling degree of the above-mentioned thin film was calculated in exactly the same manner as in reference example 1, except that the electrolyte salt was dissolved at a concentration of 2.0mol/L in a mixed solvent in which ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate were mixed at a volume ratio of 25:35: 40. The results are shown in Table 1.

[ reference example 5]

In this reference example, ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate were mixed at a volume ratio of 40:30:30, and LiPF as an electrolyte salt was dissolved at a concentration of 1.0mol/L₆The swelling degree of the film was calculated in exactly the same manner as in reference example 1 except for the electrolyte solution (b). The results are shown in Table 1.

[ Table 1]

	Degree of swelling (%)
		Reference example 1	16
Reference example 2	16
		Reference example 3	15
Reference example 4	12
		Reference example 5	18

As is apparent from table 1, the electrolyte solutions of reference examples 1 to 4, which contained a solvent having a viscosity of 2.5mPa · s or less at 20 ℃ in an amount of 70 to 80 vol% of the entire electrolyte solution excluding the electrolyte salt, exhibited lower swelling degrees of the above-mentioned thin film composed of only carboxymethyl cellulose (CMC) than the electrolyte solution of reference example 5, which contained a solvent having a viscosity of 2.5mPa · s or less at 20 ℃ in an amount of 60 vol% of the entire electrolyte solution excluding the electrolyte salt. As a result, when the solvent having a viscosity of 2.5mPa · s or less at 20 ℃ is contained in an amount of 70 to 80 vol% of the entire electrolyte excluding the electrolyte salt, swelling of the binder can be suppressed, clogging can be prevented, and the lithium ion conductivity can be improved, which contributes to improvement of the discharge rate characteristic.

[ example 8 ]

In this example, first, Li as a negative electrode active material was mixed at a mass ratio of 98:1₄Ti₅O₁₂(TLO), Styrene Butadiene Rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener to prepare a slurry for a high resistance layer. Next, a separator 6 was formed in exactly the same manner as in example 1, except that the slurry for a high resistance layer was applied to the surface of the polyethylene film to form a high resistance layer 7, and the polyethylene film on which the high resistance layer 7 was formed was punched out into a disc shape having a diameter of 19.5 mm. The volume resistivity of the high-resistance layer 7 was measured to be 3.6 × 10⁴Ωcm。

Next, a button cell type lithium ion secondary battery 1 was produced in exactly the same manner as in example 1, except that the separator 6 obtained in this example was used and the separator 6 was disposed so that the high-resistance layer 7 was formed on the negative electrode mixture layer 5 side.

Next, a charge and discharge test was performed in exactly the same manner as in example 1, except that the lithium-ion secondary battery 1 obtained in this example was used. The cycle performance is shown in FIG. 5, and the capacity retention rate is shown in FIG. 6.

[ comparative example 3]

In this comparative example, a button cell type lithium ion secondary battery 1 was obtained in the same manner as in example 8, except that the high-resistance layer 7 was not formed at all.

Next, a charge and discharge test was performed in exactly the same manner as in example 1, except that the lithium-ion secondary battery 1 obtained in this comparative example was used. The cycle performance is shown in FIG. 5, and the capacity retention rate is shown in FIG. 6.

As is apparent from fig. 5, the lithium ion secondary battery 1 of example 8 having the high-resistance layer 7 on the surface of the separator 6 on the negative electrode mixture layer 5 side has an improved capacity as compared with the lithium ion secondary battery 1 of comparative example 3 having no high-resistance layer 7.

As is apparent from fig. 6, the lithium ion secondary battery 1 of example 8 having the high-resistance layer 7 on the surface of the separator 6 on the negative electrode mixture layer 5 side has a higher capacity retention rate than the lithium ion secondary battery 1 of comparative example 3 having no high-resistance layer 7 at all.

[ example 9 ]

In this example, LiPF as an electrolyte salt was dissolved at a concentration of 1.0mol/L in a mixed solvent in which ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate were mixed at a volume ratio of 10:45:45₆A stacked lithium ion secondary battery 1 was obtained in exactly the same manner as in example 3, except that the electrolyte solution was used.

In the lithium ion secondary battery 1 of the present example, ethyl methyl carbonate and dimethyl carbonate as solvents having a viscosity of 2.5mPa · s or less at 20 ℃ were contained in an amount of 90% by volume in total of the above electrolyte solutions excluding the electrolyte salt.

Next, a charge and discharge test was performed using the lithium ion secondary battery 1 produced in this example. In the above charge and discharge test, the operation of charging to 4.15V at 1C and discharging to 2.5V at 2C was defined as 1 cycle, and the operation was repeated for 50 cycles, whereby the capacity retention rate with respect to the number of charge and discharge cycles was measured. The results are shown in FIG. 7.

[ example 10 ]

In this example, a mixed solvent as an electrolyte solution in which ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate were mixed at a volume ratio of 25:35:40 was added to prepare an electrolyte salt of LiPF at a concentration of 1.0mol/L₆A stacked lithium ion secondary battery 1 was produced in the same manner as in example 3, except that the electrolyte solution of (a) was dissolved in the mixed solvent.

In the lithium ion secondary battery 1 of the present example, ethyl methyl carbonate and dimethyl carbonate as solvents having a viscosity of 2.5mPa · s or less at 20 ℃ were contained in an amount of 75% by volume in total of the above electrolyte solutions excluding the electrolyte salt.

Next, a charge/discharge test was performed in exactly the same manner as in example 9, except that the lithium-ion secondary battery 1 obtained in this example was used. The results are shown in FIG. 7.

[ example 11 ]

In this example, LiPF as an electrolyte salt was dissolved at a concentration of 1.0mol/L in a mixed solvent in which ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate were mixed at a volume ratio of 35:30:35₆A stacked lithium ion secondary battery 1 was produced in the same manner as in example 3, except that the electrolyte solution was used as a mixed solvent for the electrolyte solution.

In the lithium ion secondary battery 1 of the present example, ethyl methyl carbonate and dimethyl carbonate as solvents having a viscosity of 2.5mPa · s or less at 20 ℃ were contained in an amount of 65% by volume in total of the above electrolyte solutions excluding the electrolyte salt.

Then, a charge and discharge test was performed in exactly the same manner as in example 9, except that the lithium-ion secondary battery 1 obtained in this example was used. The results are shown in FIG. 7.

[ example 12 ]

In this example, ethylene carbonate, ethyl methyl carbonate, dimethyl carbonate, and bis (2, 2, 2-trifluoro-2) were mixed in a volume ratio of 20:30:20:30Ethyl) ether was dissolved in a mixed solvent containing LiPF as an electrolyte salt at a concentration of 1.0mol/L₆A stacked lithium ion secondary battery 1 was produced in the same manner as in example 3 except for the solution as an electrolytic solution.

In the lithium-ion secondary battery 1 of the present example, ethyl methyl carbonate, dimethyl carbonate, and bis (2, 2, 2-trifluoroethyl) ether as a solvent having a viscosity of 2.5mPa · s or less at 20 ℃ were contained in an amount of 80% by volume in total of the electrolyte solution excluding the electrolyte salt.

Next, a charge and discharge test was performed in exactly the same manner as in example 9, except that the lithium-ion secondary battery 1 obtained in this example was used. The results are shown in FIG. 7.

[ example 13 ]

In this example, LiPF as an electrolyte salt was dissolved at a concentration of 1.0mol/L in a mixed solvent in which ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate were mixed at a volume ratio of 5:45:50₆A stacked lithium ion secondary battery 1 was produced in the same manner as in example 3, except that the electrolyte solution was used.

In the lithium ion secondary battery 1 of the present example, ethyl methyl carbonate and dimethyl carbonate as solvents having a viscosity of 2.5mPa · s or less at 20 ℃ were contained in an amount of 95% by volume in total of the above electrolyte solutions excluding the electrolyte salt.

Next, a charge/discharge test was performed in exactly the same manner as in example 9, except that the lithium-ion secondary battery 1 obtained in this example was used. The results are shown in the figure.

[ example 14 ]

In this example, LiPF as an electrolyte salt was dissolved at a concentration of 1.0mol/L in a mixed solvent in which ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate were mixed at a volume ratio of 30:30:40₆A laminated lithium-ion secondary battery 1 was produced in the same manner as in example 3, except that the electrolyte solution was used as a solution for the electrolyte solution.

In the lithium ion secondary battery 1 of the present example, ethyl methyl carbonate and dimethyl carbonate as solvents having a viscosity of 2.5mPa · s or less at 20 ℃ were contained in an amount of 70% by volume in total of the above electrolyte solutions excluding the electrolyte salt.

[ example 15 ]

In this example, LiPF as an electrolyte salt was dissolved at a concentration of 1.0mol/L in a mixed solvent in which ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate were mixed at a volume ratio of 60:20:20₆A laminated lithium-ion secondary battery 1 was produced in the same manner as in example 3, except that the electrolyte solution was used as a solution for the electrolyte solution.

In the lithium ion secondary battery 1 of the present example, ethyl methyl carbonate and dimethyl carbonate as solvents having a viscosity of 2.5mPa · s or less at 20 ℃ were contained in an amount of 40% by volume in total of the above electrolyte solutions excluding the electrolyte salt.

As is apparent from fig. 7, according to the lithium ion secondary batteries 1 of examples 9 to 12 and 14 containing the solvent having the viscosity of 2.5mPa · s or less at 20 ℃ in an amount of 70 to 90 vol% of the entire electrolyte solution excluding the electrolyte salt, the capacity retention rate is higher than that of the lithium ion secondary battery of example 13 containing the solvent in an amount of 95 vol% of the entire electrolyte solution excluding the electrolyte salt and that of the lithium ion secondary battery 1 of example 15 containing the solvent in an amount of 40 vol%. The reason why the capacity retention rate of the lithium-ion secondary battery 1 of example 13, which contains the solvent having a viscosity of 2.5mPa · s or less at 20 ℃ in an amount of 95 vol% or more of the entire electrolyte solution excluding the electrolyte salt, is lower than the capacity retention rate of the lithium-ion secondary batteries 1 of examples 9 to 12 and 14 is considered to be that dissociation of the lithium salt in the electrolyte solution hardly proceeds.

[ description of symbols ]

1 … lithium ion secondary battery, 3 … positive electrode mixture layer, 5 … negative electrode mixture layer, 6 … diaphragm, 7 … high resistance layer.

Claims

1. A lithium ion secondary battery comprising a positive electrode mixture layer, a negative electrode mixture layer, and a separator interposed between the positive electrode mixture layer and the negative electrode mixture layer,

a high-resistance layer containing a positive electrode active material and a binder or a negative electrode active material and a binder and having a volume resistivity of 2500 Ω cm or more is provided between the positive electrode mixture layer and the separator or between the negative electrode mixture layer and the separator, the high-resistance layer having a volume resistivity different from that of the positive electrode mixture layer or the negative electrode mixture layer,

the electrolyte solution impregnated into the separator contains a solvent having a viscosity of 2.5 mPas or less at 20 ℃ in an amount within a range of 60 to 95 vol% of the entire electrolyte solution excluding the electrolyte salt.

2. The lithium-ion secondary battery according to claim 1,

the high-resistance layer is provided on the surface of the positive electrode mixture layer or the negative electrode mixture layer on the separator side.

3. The lithium-ion secondary battery according to claim 1,

the separator is provided with the high-resistance layer on a surface on the positive electrode mixture layer side or the negative electrode mixture layer side.

4. The lithium-ion secondary battery according to claim 1,

the electrolyte solution impregnated into the separator contains a solvent having a viscosity of 2.5 mPas or less at 20 ℃ in an amount within a range of 65 to 90 vol% of the entire electrolyte solution excluding the electrolyte salt.

5. The lithium-ion secondary battery according to claim 1,

the electrolyte solution impregnated into the separator contains a solvent having a viscosity of 2.5 mPas or less at 20 ℃ in an amount within a range of 70 to 90 vol% of the entire electrolyte solution excluding the electrolyte salt.

6. The lithium-ion secondary battery according to claim 1,

the electrolyte is in a gel state.

7. The lithium-ion secondary battery according to claim 1,

the electrolyte contains electrolyte salt with the concentration range of 0.1-3.5 mol/L.

8. The lithium-ion secondary battery according to claim 1,

the electrolyte contains electrolyte salt with the concentration range of 1.0-2.5 mol/L.